Most conventional image sensors operate by sensing an image projected onto an array of discrete, physically separated image sensor elements. The electrical response of each image sensor element is proportional to the total light falling within its boundaries. The electrical pattern held by the array of discrete image sensor elements is retrieved by sequentially interrogating the electrical response of each of the sensor elements. Currently, the light sensitive areas of the individual detector elements are designed to be discrete, with light falling within one detector element only inducing an electrical response in that particular light illuminated detector element.
Isolation between neighboring detector elements is commonly achieved by physically patterning the active region of each detector element so that it is isolated from its neighbors. However, physical isolation of detector elements has a number of drawbacks. For example, the isolation of the detector elements requires additional patterning and photolithographic steps. At least one, and possibly more, mask steps must be devoted to ensuring that the detector elements remain isolated from each other.
Another problem associated with patterning techniques for detector element isolation is that the aperture or active area of a pixel is already limited by the space allocated to signal lines and pass transistors. Indeed, the active area fraction of a pixel is significantly less than one, being as low as 30-40 percent for high resolution image sensors. If the surface area devoted to physical isolation of detector elements can be reduced, economy and yield can be increased while maintaining the same signal to noise ratio.
The use of physical patterning for detector element isolation can lead to undesirable artifacts in scanned images. For example, aliasing patterns, also known as moire effects, result from interference between periodic structures in the image and the inherent periodicity of the patterned detector elements. This interference causes spurious periodic features to appear in a scanned image that are not present in the original image. Unfortunately, the aliasing patterns can not be removed by image processing techniques because it is not known a priori whether a given spatial pattern is an unwanted result of aliasing, or an actual pattern that exists in the image.
Aliasing problems in an image can be reduced by several techniques, with a decrease in detector element size and spacing being the most straightforward. If the detector element spacing frequency is greater than twice the spatial frequency of periodic image features, aliasing can generally be avoided. However, physical limitations in current detector fabrication methods, coupled with the increased cost of an array of very small detector elements, and the high frequency of periodic spatial image detail desired for high resolution sensor systems, place limits on widespread implementation of this solution. Another potential solution to aliasing problems relies on decreasing the gap distance between detector elements. However, there is again increased difficulty and cost associated with fabrication of a gapless detector element array. In addition, even if the gap distance is reduced to zero, high spatial frequency periodic image features (those with a frequency greater than twice the frequency of detector element spacing) will still cause aliasing problems. More effective anti-aliasing techniques are required for fabrication of inexpensive and reliable high resolution systems with minimal image aliasing,
The present invention minimizes aliasing problems by providing for fabrication of an array of detector elements with overlapping responsive areas. Because the responsive areas can be made larger than the spacing between the detector elements, aliasing and moire effects can be greatly attenuated. In addition, the responsivity of the overlapping detection areas utilized in the present invention changes smoothly to zero, eliminating high frequency sidelobes (with its resultant aliasing) associated with conventional non-continuous sharp edged detector elements. This is accomplished by forming an image sensor array having a plurality of detection zones for producing photogenerated charge, with each detection zone centered on a charge collection element for holding charge, and with local detection response in each detection zone smoothly decreasing with distance from each charge collection element.
In preferred embodiments, the present invention is an image sensor array having overlapping responsive zones. These overlapping responsive zones are respectively centered on a plurality of collection electrodes, and utilize a photosensitive or radiation sensitive layer (e.g., a layer that responds to photonic radiation, including visible light, ultraviolet, x-rays, or non-photonic sources such as charged particles) in contact with the collection electrodes to generate photoinduced charge. Preferably, the photosensitive layer (which hereinafter is defined to include all energetic forms of radiation) is configured to distribute locally photogenerated charge to more than one collection electrode.
To improve ease of measuring the photoinduced charge, a plurality of pass transistors can be connected to receive charge from the collection electrodes. In certain embodiments field effect pass transistors are used, with each pass transistor having a drain connected to one of the collection electrodes, a source separated from the drain, and a pass transistor gate electrode controllable to promote passage of photoinduced charge from the drain to the source. Alternatively, a pass transistor can be arranged so that the photoinduced charge controls external current passing between a drain and a source, an arrangement which advantageously allows for signal amplification at the pass transistor. The present invention can also be used for color applications with the addition of more photosensitive layers.
In a most preferred embodiment, an image detector array in accordance with the present invention includes a plurality of collection electrodes organized in a one or two dimensional array. A continuous photosensitive layer in contact with each of the collection electrodes is configured to produce a detectable response at the collection electrode upon incidence of radiation (usually, but not limited to light) in a responsive zone. To minimize aliasing, the image detector array is arranged to present overlapping radiation responsive zones associated with each of the collection electrodes. A real extent of the overlap is controlled by adjustments to collection electrode resistivity. Typically, the collection electrode is constructed from an n+ doped amorphous silicon layer, and is adjacent to the photosensitive layer, an intrinsic amorphous silicon layer, which in turn is adjacent to a p+ doped layer, forming a p-i-n diode.
Other objects and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the drawings and preferred embodiments.